1. Field of the Invention
This invention relates to a PN code generation apparatus and method, and a radio communication apparatus, and more particularly, is suitably applied to a cellular radio communications system according to the code division multiple access (CDMA) system.
2. Description of the Related Art
In the cellular radio communications system, an area for offering the communication service is divided into cells with the desired sizes and a base station as the fixed radio station is provided in each cell and a portable telephone as a mobile radio station is connected by radio to the base station having the optimal communication condition.
In this CDMA system which is one of the mobile communication systems, the transmitting side allocates specific pseudo-random noise (PN) sequence code formed of pseudo-random noise sequence code to each communication circuit with the same carrier frequency and by multiplying the primary modulation wave by the PN code, spreads this to wider band than the original frequency band (hereinafter referred to as spread spectrum) and transmits the secondary modulation wave supplied the spread-spectrum processing.
The mobile station of cellular radio communications system according to the CDMA system receives a pilot PN code transmitted through the pilot channel out of the forward link channels from the base station, and by trapping the timing (phase) of the pilot PN code, can generate the PN codes in the mobile station synchronized with the PN code to be used in the base station side.
The mobile station transmits its own position information or various kinds of information such as calls from/to its own station, to the base station through the access channel out of the rivers link channels. During conversation, a transmission symbol sequence is formed by compression-coding the communication data, such as audio information, for each frame. The transmission symbol is modulated, subjected to the spread-spectrum processing using the same PN code as that used in the base station, and is transmitted through the traffic channel out of the reverse link channels.
In the cellular radio communications system according to the CDMA system, in order to synchronize times among all base stations, each base station sets the absolute reference time using the global positioning system (GPS) wave according to the IS-95 standard and thus, time synchronization among base stations is obtained.
In this case, each base station. transmits the same long code PN code, as the spread code based on the absolute reference time at different timing for each base station to base station, and thus, by trapping only the reception timing (phase) of the long code PN code, the mobile station can search the base station to be connected and simultaneously can obtain the system time synchronized with the absolute reference time.
With this arrangement, the mobile station forms pilot PN code synchronized with the system time of the base station with the pilot PN code generator to be described later, and multiplying the transmission symbol sequence by this, applies the spread-spectrum processing. At this point, in the mobile station, in the case of multiplying transmission symbol sequence by the pilot PN code, it is necessary that the timing of the start time of the transmission frame (the border between transmission frames) coincides with the timing of the pilot PN code generation.
As shown in FIG. 1, in the mobile station, mark per 2-second of the system time obtained by synchronizing with the connecting base station and the start time of the transmission frame (the border between the transmission frame of the frame number 0 and the transmission frame of the frame number 3) are in agreement. This is regulated in the IS-95. Moreover, since the pilot PN code, which is generated by the pilot PN code generator, is the 15th-order PN code and it has the chip rate of 1.2288 Mbps, and its cycle is 26.66 . . . msec (215/122880 sec).
With this arrangement, three cycles of the pilot PN code (26.66 . . . msecxc3x973) and one cycle of the super frame (80 msec) formed of four frames of transmission frame are in agreement. Since one cycle of the pilot PN code is 26.66 msec, it repeats 75 cycles (2/0.02666) by the time the next mark per 2 second appears.
As shown in FIG. 2, in the conventional pilot PN code generator 1 for generating these pilot PN codes, the reference clock CLK synchronized with the absolute reference time is supplied and also timing information for each the start time point of the super frame SFRM is supplied. And the pilot PN code generator 1 generates the 15th-order pilot PN code based on the reference clock CLK and the timing information SFRM.
In practice, the pilot PN code generator 1 loads an initial value SINIT memorized in the internal memory 2 in advance, based on the timing information SFRM synchronized with the reference clock CLK, and based on the initial value SINIT, initializes the whole pilot PN code generator 1 only at each start time of the super frame or at the operation start time.
Accordingly, since the pilot PN code generator 1 is initialized based on the initial value SINIT, the mobile station is able to conduct the spread-spectrum processing by generating pilot PN codes from the head of the code sequence pattern at the timing of the super frame.
Accordingly, even in the case where the control information was received from the base station and response information should be transmitted before the start time of the frame number 2 of the super frame, since the mobile station could not obtain the initial value SINIT until the next super frame timing, the mobile station could not transmit the response information. More specifically, the mobile station could not generate pilot PN codes which is synchronized with the super frame, to be multiplied by the transmission symbol sequence at the start point timing of the frame number 2, and it caused a problem that the waiting time became 80 msec at the longest and the response information could not be transmitted in time.
In view of the foregoing, an object of this invention is to provide a PN code generation apparatus and method, and a radio communication apparatus which can generate PN codes synchronized with a super frame at the start timing of an arbitrary transmission frame from the head of code sequence pattern.
The foregoing object and other objects of the invention have been achieved by the provision of a radio communication apparatus for generating a transmission signal subjected to the spread spectrum by multiplying a PN code sequence by transmission data, in which a control means controls timing of sequentially generating a plurality of transmission frames as transmission data, and a PN code generating means generates a PN code sequence having n cycles (n is an integral number more than 1 and is not m) which has a synchronization relation with the transmission frame of m cycle (m is an integral number more than 1). In the case of transmitting a response signal to a receive signal, the control means supplies identification information and start timing information of a specific frame out of the plurality of transmission frames, and the PN code generating means starts generating the PN code sequence keeping the synchronization relation with the transmission frame at the start timing of the specific frame in response to the identification information and the start timing information of the specific frame.
The nature, principle and utility of the invention will become more apparent following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.